1. Field of the Invention
The present invention relates a variable attenuator for use in an SPST (Single-Pole Single-Throw) switch, an SPDT (Single-Pole Double-Throw) switch, etc, which conducts or blocks a high-frequency signal and, more particularly, to a variable attenuator which provides a variable attenuation to a high-frequency signal during conduction.
2. Description of the Related Art
Known as a high-frequency switch for conducting or blocking a high-frequency signal are resonance type high-frequency switches (as disclosed in Japanese Unexamined Patent Application Publication No. 2000-114950 and its corresponding U.S. Pat. No. 6,281,762), which include a field-effect transistor (hereinafter referred to as an FET), an inductive element, a capacitive element, etc. Such a high-frequency switch switches between a parallel resonance state and a serial resonance state in synchronization with the on and off operation of the FET by appropriately setting inductance of the inductive element and capacitance of the capacitive element. In this way, the conventional high-frequency switch conducts a high-frequency signal at the serial resonance state thereof and blocks the high-frequency signal at the parallel resonance state thereof.
When a variable attenuator for attenuating the high-frequency signal is mounted on the high-frequency switch, an FET for attenuation, separately from the FET forming the high-frequency switch, is connected to the high-frequency switch. In this way, a voltage applied between the gate and the source of the attenuator FET is controlled to control the amount of attenuation of the high-frequency signal conducted between the drain and the source.
In the above conventional art, the high-frequency switch and the variable attenuator are separately produced. For this reason, the entire unit becomes bulky when the high-frequency switch and the variable attenuator are produced on an MMIC (Monolithic Microwave Integrated Circuit) fabricated of an expensive material such as GaAs, and the manufacturing cost of the switch increases. Since the amount of attenuation is set by the attenuating FET only, a large amount of attenuation cannot be set.
The present invention has been developed in view of the above problem and it is an object of the present invention to provide a variable attenuator which sets a large amount of attenuation, involves a low manufacturing cost, and has a compact overall size.
To resolve the above problem, a variable attenuator of the present invention in one aspect includes an FET, an inductive element connected to one of the source and the drain of the FET, a capacitive element connected in parallel with the series connection of the inductive element and the FET, a first terminal and a second terminal respectively connected to both ends of the capacitive element, and an attenuation setting unit which variably sets the amount of attenuation to a signal transmitted across the first terminal and the second terminal by varying the gate voltage of the FET.
In this arrangement, the circuit between the two terminals is broken when the FET is turned on, and the circuit between the two terminals is made when the FET is turned off. With the FET set in the vicinity of pinchoff to achieve a conductive state between the two terminals, the attenuation setting unit variably sets the amount of attenuation.
Since the attenuator is formed of a resonance type SPST switch including the FET, the inductive element, and the capacitive element, the attenuator has a wide range of attenuation and a large amount of attenuation compared with the case in which an attenuator is formed of an FET alone.
Preferably, the variable attenuator further includes a constant-voltage source, connected to one of the source and the drain of the FET, for supplying a constant voltage, wherein the attenuation setting unit is a variable-voltage generator which is connected to the gate of the FET to variably set the gate voltage of the FET.
The voltage difference between the constant voltage provided by the constant-voltage source and the voltage provided by the variable-voltage generator is applied to the FET as a gate voltage (a gate-source voltage or a gate-drain voltage), thereby variably setting the amount of attenuation between the two terminals.
Preferably, the gate of the FET is grounded, and the attenuation setting unit is a variable-voltage generator which is connected to one of the source and the drain of the FET to variably set the voltage of one of the source and the drain of the FET.
The voltage difference between the ground voltage and the voltage provided by the variable-voltage generator is applied to the FET as the gate voltage, thereby variably setting the amount of attenuation between the two terminals. For this reason, the amount of attenuation is set by connecting the single variable-voltage generator to the one of the source and the drain of the FET.
A variable attenuator of the present invention in another aspect includes a first FET, a first inductive element connected to one of the source and the drain of the first FET, a first capacitive element connected in parallel with the series connection of the first inductive element and the first FET, a second FET, a second inductive element connected to one of the source and the drain of the second FET, a second capacitive element connected in parallel with the series connection of the second inductive element and the second FET, a first terminal connected to each end of the first capacitive element and the second capacitive element, a second terminal connected to the other end of the first capacitive element, a third terminal connected to the other of the second capacitive element, a voltage switch which makes or breaks the circuit between the first terminal and the third terminal by varying the gate voltage of the second FET, and an attenuation setting unit which variably sets the amount of attenuation to a signal transmitted across the first terminal and the second terminal by varying the gate voltage of the first FET.
In this arrangement, the circuit between the first terminal and the third terminal is broken when the voltage switch turns on the second FET, and the circuit between the first terminal and the third terminal is made when the voltage switch turns off the second FET. The circuit between the first terminal and the second terminal is broken when the attenuation setting unit turns on the first FET, and the circuit between the first terminal and the second terminal is made when the attenuation setting unit turns off the first FET. In this way, the signal transmitted across the two terminals is attenuated by causing the attenuation setting unit to operate the first FET in the vicinity of pinchoff during the conductive state of the attenuator.
A variable attenuator of the present invention in yet another aspect includes one FET, an inductive element connected to one of the source and the drain of the one FET, a capacitive element connected in parallel with the series connection of the inductive element and the one FET, another FET with one of the source and the drain thereof connected to the node of the capacitive element and the one FET, a first terminal connected to the node of the capacitive element and the one FET, a second terminal connected to the other end of the capacitive element opposite from the first terminal with respect to the capacitive element, a third terminal connected to the other end of the other FET opposite from the first terminal with respect to the other FET, and an attenuation setting unit which variably sets the amount of attenuation to a signal transmitted across the first terminal and the second terminal by varying the gate voltage of the two FETs.
In this arrangement, the circuit between the first terminal and the second terminal is broken and the circuit between the first terminal and the third terminal is made when the attenuation setting unit causes the two FETs to turn on. The circuit between the first terminal and the second terminal is made and the circuit between the first terminal and the third terminal is broken when the attenuation setting unit causes the two FETs to turn off.
A resonance type SPST switch is formed of the FET, the inductive element, and the capacitive element. The attenuation setting unit operates the FET of the SPST switch in the vicinity of pinchoff, and thus the amount of attenuation between the first terminal and the second terminal is variably set. When the SPST switch operates in coordination with the other FET, the attenuation setting unit increases the impedance of the SPST switch, thereby reducing the impedance of the other FET. When the attenuation setting unit reduces the impedance of the SPST switch, the impedance of the other FET is increased. This arrangement controls variations in the impedance with respect to the first terminal.
Preferably, the variable attenuator further includes another inductive element connected between the drain and source of the other FET in parallel with the other FET.
When the other FET is turned off, the off capacitance of the other FET and the other inductive element resonate in parallel, thereby increasing the isolation between the first terminal and the third terminal.
Preferably, the gates of the two FETs are grounded, and the attenuation setting unit is a variable-voltage generator which is connected to the first terminal for connecting the two FETs and variably sets the voltage of one of the source and the drain of each of the two FETs.
In this way, the voltage difference between the ground voltage and the voltage of the variable-voltage generator is supplied to the two FETs, thereby variably setting the amount of attenuation between the first terminal and the second terminal. For this reason, the amount of attenuation is set by connecting the single variable-voltage generator to the sources or the drains of the two FETs.
Preferably, one of the second terminal and the third terminal is grounded, and an SPST switch is formed between the other of the second terminal and the third terminal, and the first terminal.
When the third terminal is grounded, for example, the circuit between the first terminal and the third terminal is broken while the circuit between the first terminal and the second terminal is made. The circuit between the first terminal and the third terminal is made while the circuit between the first terminal and the second terminal is broken, and the first terminal is grounded. For this reason, the degree of isolation between the conductive state between the first and second terminals and the non-conductive state between the first and second terminals increases thereby widening a range in which attenuation varies.
Preferably, an SPDT switch is formed of a plurality of SPST switches in series connection.
Since the SPDT switch is formed from a plurality of SPST switches, power transmitted thereacross is increased.
A variable attenuator of the present invention in still another aspect includes a plurality of SPST modules in parallel connection, an SPST module including an FET, an inductive element connected to one of the source and the drain of the FET, a capacitive element connected in parallel with the series connection of the inductive element and the FET, and a first terminal and a second terminal respectively connected to both ends of the capacitive element, and an attenuation setting unit for variably setting the amount of attenuation on a signal transmitted across the first terminal and the second terminal by varying the gate voltage of the FET.
Since the power transmitted across the terminals is increased, a larger power is attenuated.